The problems of propagating radio signals into underground regions have been evident since the earliest days of radio communication. For example, it was soon found, for the frequencies used, that the radio signals did not penetrate into tunnels beyond a few hundred feet. Thus, communication with trains when they were within tunnels of any substantial length was not possible with the then existing radio systems.
One of the first successful tunnel communications systems is that described in N. Monk and H. S. Winbigler, "Communication with Moving Trains in Tunnels," IRE Trans. Vehic. Comm., Vol. VC-7, Dec. 1956, pp. 21-28, in which the leakage fields of the standard coaxial cable running through a railroad tunnel were found to be adequate to provide communication between a fixed base station connected to one end of the cable and a radio receiver on a train passing through the tunnel.
The "leaky feeder" principle, using the leakage from an open-braided type of coaxial cable has been used in numerous subsequent installations. Another type of cable, in widespread use, utilized a corrugated-outer type of cable but with discrete holes or slots milled in an otherwise solid shield to provide the necessary radio frequency leakage.
Although successful in use, the leaky-feeder coaxial cables have several disadvantages. The cables are often quite stiff, and should be spaced away from metal or concrete surfaces, requiring attachments spaced approximately five feet apart along the length of the cable. The consequent cost of installation is quite high because of the large amount of labor required.
In subways or other tunnels, the leaky coaxial cable is usually mounted high on the tunnel wall, where it is vulnerable to melting in the event of a fire, or to damage in case of derailment.
In addition, the input power into the cable must be relatively great in order to provide sufficient effective radiated power for communication. The radiated power decreases significantly along the length of the cable, requiring numerous radio frequency amplifiers along the length of the cable to maintain the signal power in the cable at a high enough level for effective communication.
In 1981, a distributed antenna system was developed to overcome some of the disadvantages of the leaky-feeder systems. The system, described in R. A. Isberg, J. C. Cawley and R. L. Chufo, "The Design and Implementation of a VHF Radio System Using Distributed Antennas, Passive Reflectors and Two-Way Signal Boosters in a Room and Pillar Limestone Mine," IEFE 32nd Vehicular Technology Conference Record, May 1982, used a 1200 foot long coaxial cable which fed, through 2 to 1 power dividers, four whip antennas spaced along the length of the cable. The cost of installation was significantly less than a leaky coaxial cable installation would have been, and it was found that the distributed antenna system had a substantially greater operating range for the same amount of input power than would have been required for a leaky coaxial cable system. For the 1200 foot cable system, with four antennas, the effective radiated power from the various antennas ranged from 17 watts for the antenna nearest to the base station end of the cable to 0.05 watts for the antenna farthest from the base station. The signals from transceivers received at the most distant antennas were attenuated approximately 21 dB by losses in the cable and power dividers.
A subsequent distributed antenna system, described in R. A. Isberg and D. Turrell, "Applying CATV Technology and Equipment in Guided Radio Systems," IEEE 34th Vehicular Technology Conference Record, May, 1984, used two coaxial cables, one for transmission and the other for reception, extending through tunnels, with UHF antennas connected to each cable and spaced 75 to 100 meters apart along the cables. The transmitting antennas were located closely adjacent the receiving antennas, and the antennas were connected to the cables by two-way power splitters each with a 4 dB loss. CATV amplifiers were used at intervals along the length of the cables to maintain the signal power in the cables at an effective level, with one or two antennas being coupled to the cables between successive amplifiers.
A more recent distributed antenna system is described in R. A. Isberg, R. Trottier and B. Hicks, "A Guided Radio System Using CATV Amplifiers and Pressure Taps on CATV Amplifiers and Pressure Taps on CATV Cable to Feed Distributed Antennas," IEEE 35th Vehicular Technology Conference Record, May, 1985, wherein, for example, a single 420 foot long CATV coaxial cable was extended through a ship's alleyway, with UHF one quarter wave length mobile whip antennas being connected to the cable at 50 foot intervals by standard CATV 12 dB isolation pressure taps. Such a tap has a pointed center conductor which extends through a hole in the braided shield of the cable to contact the center conductor of the cable. A type F receptacle on the tap is coupled to the center conductor through a small capacitor A ferrite transformer and resistor in the tap couple the radio frequency signal from the cable to the antenna or vice versa.
Although the distributed antenna systems that have been installed overcome some of the disadvantages of the leaky cable systems, they have the same disadvantage in that the power to the various antennas along the length of the cable decreases substantially from the end of the cables to which the radio signal is applied. Likewise, the total attenuation of a signal received at an antenna farthest from the radio receiver end of the cable will be much greater than that of a signal received from the antenna nearest the radio receiver and of the cable. As a consequence a considerable number of signal booster amplifiers are required to maintain the effective radiated power at a sufficient level at all antennas, and to overcome the different total attenuation of signals received by the different antennas from transceivers in communication with the system. This problems is exacerbated in situations wherein governmental regulations restrict the amount of radiated power from the transmitting system or the transceivers to a low level. For example, the FCC Rules and Regulations applicable to ship-board installations limit the transmitter output power to four watts, and the effective radiated power to two watts for on-board communication on 457.525, 457.550, 457.575 and 457.600 mHz channels.
It has also been found in the prior distributed antenna systems that interfering spurious or intermodulation signals generated by the transmitter or signal booster amplifiers will be present at the radio receiver along with the signals from the transceiver, and it has been difficult and expensive to attenuate those interfering signals.